Every year more than one million balloon angioplasty procedures are performed in the United States. These medical procedures are carried out in the abdominal cavity and in the arteries, using catheters and miniature ultrasonic instrumentation. Most of these interventional techniques use X-ray guidance, which requires expensive, non-mobile facilities, and use expensive and sometimes harmful "contrast" materials that produce only a projection image.
In contrast to the X-ray techniques, ultrasound guidance can be simpler and less expensive. It does not require a contrast media and provides three-dimensional information about the tissue structure. However, ultrasound guidance has not been used, until recently, because it requires visualization of a particular point on the catheter. The catheter itself has a high ultrasound reflectivity and visualization of it depends on the angle of the incident ultrasound beam. In order to solve this problem and make the appearance of a point on the catheter both independent of the angle and of the surrounding tissue reflectivity, Vilkomersan and co-workers (D. Vilkomersan et al., "Quasi-omnidirectional transducers for ultrasonic electronic-beacon guidance of invasive devices", SPIE, Vol. 1733, p.154-165, 1992) proposed a quasi-omnidirectional annular shaped ultrasonic sensor located on the catheter. Due to the difficulties in fabricating such a transducer from ceramic piezoelectrics, Vilkomersan et al. used a PVF2 copolymer as the ultrasonic sensor. However, it is known to those skilled in the art that polymer based piezoelectrics are associated with high dielectric losses and low electromechanical coupling coefficients and low dielectric constant.
An electroactive ceramic with low losses and high electromechanical coupling coefficients and high dielectric constant would be more suitable for the ultrasonic guidance of a catheter. However, machining a quasi-omnidirectional annular shaped ultrasonic sensor from a fragile ceramic, with current techniques, is not feasible. Also to be considered are the health concerns and hygienic requirements associated with catheters.
Catheters are discarded after one use to prevent spreading of blood-borne diseases. Considering the high volume of use of these interventional techniques, there is need for small, inexpensive transducers for use in assisting the guidance of catheters.
In clinical applications of ultrasonic transducers, such as Ultrasonic Backscatter Microscopy and Ultrasonic Imaging Catheters, higher axial and lateral resolution requires operating frequencies greater than 20 MHz. In recent years transducers operating in this range have been prepared from piezoelectric polymers as well as from piezoelectric ceramics. See: M. S. Shearer et al., "The design and fabrication of high frequency poly (vinylidene fluoride) transducers," Ultrasonic Imaging, vol. 11, pp. 75-94, 1989, and F. S. Foster et al., "Characterization of lead zirconate titanate ceramics for use in miniature high-frequency (20-80 MHz) Transducers," IEEE Trans. Ultrason. Ferroelec. Freq. Contr., vol. 38, no.5, pp. 446-453, 1991.
Polymer-based transducers have good beam properties, broad bandwidth, and can be fabricated easily into various shapes, but as stated above, they are also associated with high losses and low electromechanical coupling coefficients. Ceramic transducers, with their high coupling coefficients and low losses, lead to improved image qualities.
However, obtaining a high resolution image requires a focused ultrasound beam. Using an electronically steerable array of transducers to obtain a focused beam is not feasible due to size limitations imposed on the transducer at an operating frequency of &gt;20 MHz. Also electronic steering requires extensive use of expensive electronic equipment. Therefore, the easiest way to obtain a focused ultrasound beam requires a spherically shaped transducer. However, molding, or machining a focused transducer (wall thickness &lt;100 .mu.m due to the operating frequencies) using conventional techniques is not feasible. See: Lockwood et al. "Fabrication of high frequency spherically shaped ceramic transducers," IEEE Trans. Ultrason. Ferroelec. Freq. Contr., vol. 41, no. 2, pp. 231-235, 1994. Lockwood et al. developed a bending technique to obtain a spherical shape from very thin ceramic plates. A ceramic plate was bonded to a malleable material, such as an epoxy. This layered composite structure was shaped into a shallow spherical shell by gently pressing it against a ball bearing at 65.degree. C.
While this approach worked fairly well for high f-number transducers at very high frequencies (f-number=focal length/aperture size), it is difficult to apply the technique for transducers that operate below 40 MHz, and that have a lower f-number, where the ceramic thickness hinders the bending process. Moreover, Zipparo et al., ("Piezoceramics for high-frequency (20 to 100 MHz) single-element imaging transducers," IEEE Trans. Ultrason. Ferroelec. Freq. Contr., vol. 44, no. 5, pp. 1038-1048, 1997) reported a decrease in electromechanical coupling, dielectric constant, mechanical compliance, and an increase in mechanical losses of the ceramic, along with microcracks, resulting from this bending process.
In addition to the biomedical area, miniature ultrasonic probes have also been used for mapping the field of a hydrophone as well as the non-acoustic field of turbulent flow. There are several important requirements for microprobe sensors in these applications. In detecting underwater signals, omnidirectionality is highly advantageous. Additionally smaller sizes are necessary in some applications. Accurate mapping of an acoustic field requires that: (i) the physical dimensions of the probe should be smaller than the acoustical wavelength of interest (i.e. with a size in the millimeter range), (ii) the resonance frequencies of the probe should be well above the frequency range of interest (usually &lt;200 kHz), (iii) they should exhibit adequate sensitivity with an acceptable signal-to-noise ratio and a wide bandwidth.
Although, volume expanders with spherical shape are thought to be the best way to achieve omnidirectionality, again there are problems associated with fabricating spherical transducers with sizes in the millimeter range. There are hollow sphere shaped transducers available, commercially produced by machining and grinding hemispheres from bulk ceramics using conventional techniques. These hemispheres are then attached together using an adhesive agent. However, the machining from bulk is a laborious and expensive process. Most importantly there are limitations on the smallest size and the radius to thickness ratio, r/t, achievable through these techniques. This r/t ratio becomes important in the operation of the hollow sphere transducer and can be dubbed as the amplification factor in the calculation of figure of merit of a hollow sphere hydrophone.
Based on the above, there is a need for transducers with omnidirectional receive and transmit sensitivity, small sizes (&lt;1 cm for underwater applications and &lt;2 mm for ultrasound guidance applications) and high sensitivities. There also is a need for focused transducers that operates at high frequencies (&gt;20 MHz).
Millimeter size thin-walled hollow sphere transducers made of piezoelectric material, PZT-5 (lead zirconate titanate), were first fabricated at our laboratories several years ago (See: R. Meyer Jr. et al., "PZT hollow sphere transducers", J. Amer. Ceram. Soc., vol. 77, 6, pp. 1669-72, 1994). Green (un-fired) spheres were produced using a coaxial nozzle slurry process. The process was based on the U.S. Pat. No. 4,671,909 to Torobin and further developed in the Materials Science and Engineering Department of the Georgia Institute of Technology to mass produce ceramic and metallic hollow spheres (See A. T. Chapman et al., "Thin-wall hollow spheres from slurries", DOE-ECUT Program, ONRL Subcontract 86X-22043C, Annual Reports, 1987, 1988, 1989). Fabrication of green hollow spheres included preparation of a slurry of a fine-grained powder of the electroactive ceramic material (piezoelectric, electrostrictive, etc.) along with appropriate binders and dispersants (such as, but not limited to, poly methyl-methacrylate (PMMA), polyvinyl alcohol (PVA)) and acetone. This slurry was then injected through a coaxial nozzle with air passing through the center tube. The slurry formed a hollow cylindrical form as it left the nozzle. The bottom of the cylinder later closed with surface tension, and air pressure created a bubble.
At a certain inner pressure the bubble breaks free from the rest of the slurry, closing the upper end and forming a hollow sphere. This process has been shown to be a flexible technique for the fabrication of high volume of hollow spheres of different ceramic compositions, with fairly uniform diameters from 1 to 6 mm, and wall thicknesses ranging from 12 to 150 .mu.m. These green spheres are subsequently fired for binder burnout, and sintered at high temperature. Due to their similarity to ball bearings and to the spherical pellets used in air rifles in shape and size, electroactive ceramic hollow spheres are called "BBs" or "BB transducers".
In the prior art, sintered electroactive ceramic hollow spheres 10 were poled radially, as shown in FIG. 1 hereof, utilizing an inner electrode access hole 12, a conductive inner electrode 14 and a conductive outer electrode 16, or poled tangentially, shown in FIG. 2, utilizing external conductive electrodes located on the top 18 and bottom 20 parts of the sphere. Wire electrode leads 22 are attached to the spheres using a conductive epoxy adhesive 24.
When a potential is applied across electrodes 14 and 16, in the radial poling case, the ceramic sphere expands and shrinks volumetrically (d.sub.31 breathing vibration mode and d.sub.33 wall thickness vibration mode).
In the case of tangentially poled spheres, when a potential is applied across electrodes 14 and 16, the ceramic sphere deforms mainly in an elliptical fashion utilizing d.sub.31 & d.sub.33.
While a substantially sensitive sensor is thus obtained, there are problems associated with this design. Namely, the wires used as electrical leads do not provide a solid and compact transducer design. As a result, when used in underwater applications, the transducers tend to wobble or shift from their position in three-dimensional space. Second, a problem associated with especially the radially poled design is the epoxy adhesive used in the outer electrode wire attachment. This epoxy creates an asymmetrical mass loading on the transducer and disturbs the omnidirectional receive and transmit properties. The third problem is the acoustic impedance (Z.sub.a) mismatch between the ceramic material (Z.sub.a.apprxeq.30 MRayl) and the fluid medium (Z.sub.a.apprxeq.1.5 Mrayl) in which it will be used.
Accordingly, it is an object of this invention to provide electroactive ceramic hollow sphere transducers with omnidirectional receive and transmit response, with small size (&lt;1 cm for underwater applications and &lt;2 mm for ultrasound guidance applications).
It is another object of this invention to provide focused transducers with low f-numbers that can operate at frequencies higher than 20 MHz.
It is also an object of this invention to provide arrays and polymer-electroactive ceramic composites of these hollow sphere transducers for underwater sensing and non-destructive testing applications.
It is a further object of this invention to provide an inexpensive fabrication technique that will allow production of throwaway spherically shaped electroactive ceramic transducers.